Field of the Invention
The present invention relates to a surface light source device for illuminating a back surface of a liquid crystal display element by two types of light sources having different characteristics, thereby displaying an image on the liquid crystal display element, and a liquid crystal display device including the surface light source device.
Description of the Background Art
A liquid crystal display element provided in a liquid crystal display device does not emit light by itself. For this reason, the liquid crystal display device includes a backlight device (a surface light source device) as a light source for illuminating the liquid crystal display element at a back surface side of the liquid crystal display element.
Moreover, the liquid crystal display element includes a color filter and transmits, through a color filter, only light having a part of wavelengths in light emitted from a fluorescent lamp for emitting white light in a continuous spectrum, thereby extracting display colors of red, green and blue to perform color representation. In the case in which only light in a part of wavelength bands is cut out of light source light in the continuous spectrum to obtain the display color, thus, a transmission wavelength band of the color filter provided in the liquid crystal display element should be set to be narrower if color purity of the display color is to be increased in order to enlarge a color reproduction range. For this reason, if the color purity of the display color is to be increased, there is a problem in that a quantity of light transmitted through the color filter is decreased, resulting in reduction in a luminance.
A fluorescent lamp to be generally employed as a light source of a backlight device has a light emission spectrum with a peak in a wavelength shifted to an orange color of approximately 615 nm in a red wavelength range based on a characteristic of a phosphor. If the color purity is to be increased in a wavelength region of 630 to 640 nm which is preferable as pure red in the red color, therefore, there is a problem in that the quantity of transmitted light is extremely reduced, resulting in significant reduction in a luminance. In order to enlarge the color reproduction range while minimizing a light loss caused by the color filter, it is necessary to employ a light source for emitting light having a narrow wavelength bandwidth. In other words, it is necessary to employ a light source for emitting light having high color purity.
As a measure for solving the problems, in recent years, there has been proposed a liquid crystal display device including, as a light source, a monochromatic LED or laser having a narrow wavelength width, that is, high color purity. In particular, the laser has excellent monochromaticity and high light emission efficiency. Therefore, it is possible to obtain a liquid crystal display device having a large color reproduction area, providing an image of high quality with a high luminance, and having low power consumption.
In recent years, the light emission efficiency of the LED has also been enhanced. For this reason, a backlight device for a liquid crystal display element or the like has also used a technique for using three types of LEDs that respectively emit light having red (R), green (G), and blue (B) which are three primary colors of light and mixing colors of RGB light emitted from the three types of LEDs, thereby obtaining white light.
In order to enhance utilization efficiency of light emitted from a light source, furthermore, there has been used a directly under type backlight device for directly supplying light from a back surface to a liquid crystal display element.
In the liquid crystal display device including the backlight device, there has been desired technological development capable of providing a surface light source device having a simple structure through devise of a structural part and less luminance unevenness and color irregularity with high quality.
For example, Japanese Patent Application Laid-Open No. 2006-286906 discloses the technology for attaching a light emitting diode device for adjusting light traveling along a central axis of a lens to the lens covering a light emitting diode chip or a periphery thereof, thereby improving luminance unevenness and color irregularity in a liquid crystal display device.
The light emitting diode device described in the Japanese Patent Application Laid-Open No. 2006-286906 is designed for a divergence angle of an LED. The light emitting diode device can obtain white light without unevenness by mixture of the red LED light, the green LED light, and the blue LED light through direct adjustment of a direction of the light traveling along the central axis of the lens, that is, light traveling toward a display surface.
In contrast with a liquid crystal display device in which light sources having greatly different divergence angles are mixed, however, the way for propagation of light emitted from the light emitting diode device is varied due to a difference in light distribution possessed by the light source so that a difference is generated in the distribution of light illuminating the display surface every light source in the same light emitting diode device. In particular, a distribution difference between display surfaces having different colors causes color irregularity and is thus recognized visually.
A laser is a light source for emitting light having a narrow wavelength bandwidth. By using the laser, it is possible to enhance color reproductivity. However, the laser is a light source having a very small divergence angle, and is difficult to have equivalent expansion to LED light with a large divergence angle by a lens and the light emitting diode device through the same method as the LED. When distribution of a laser beam is different from that of the LED light over a display surface, luminance unevenness and color irregularity are caused.
In other words, in the case in which an LED light source and a laser beam source having a different light emission characteristic from that of the LED light source are to be simultaneously used in a surface light source device, it is necessary to match the ways of light expansion from the light source. It is difficult to create a uniform surface light source with the same structure. When two types of light sources are each caused to have structures for creating surface light sources, moreover, the structures are made complicated so that a manufacturing cost is increased.